From the Book

BGP Overview

BGP is an external gateway protocol, meant to be used between different networks. It is the protocol used between Internet service providers (ISPs) and also can be used between an Enterprise and an ISP. BGP was built for reliability, scalability, and control, not speed. Because of this, it behaves differently from the protocols covered so far in this book:

BGP uses the concept of autonomous systems (AS). An autonomous system is a group of networks under a common administration. The Internet Assigned Numbers Authority (IANA) assigns AS numbers: 1 to 64511 are public AS numbers and 64512 to 65535 are private AS numbers.

Autonomous systems run Interior Gateway Protocols (IGP) within the system. They run an Exterior Gateway Protocol (EGP) between them. BGP version 4 is the only EGP currently in use.

Routing between autonomous systems is called interdomain routing.

The administrative distance for EBGP routes is 20. The administrative distance for IBGP routes is 200.

BGP is a path-vector protocol. Its route to a network consists of a list of autonomous systems on the path to that network.

BGP's loop prevention mechanism is an autonomous system number. When an update about a network leaves an autonomous system, that autonomous system's number is prepended to the list of autonomous systems that have handled that update. When an autonomous system receives an update, it examines the autonomous system list. If it finds its own autonomous system number in that list, the update is discarded.

In Figure 6-1, BGP routers in AS 65100 see network 10.1.1.0 as having an autonomous system path of 65200 65300 65400.

Notification: When a problem occurs that causes a router to end the BGP peering session, a notification message is sent to the BGP neighbor and the connection is closed.

Internal and External BGP

Internal BGP (IBGP) is a BGP peering relationship between routers in the same autonomous system. External BGP (EBGP) is a BGP peering relationship between routers in different autonomous systems. BGP treats updates from internal peers differently than updates from external peers.

Before any BGP speaker can peer with a neighbor router, that neighbor must be statically defined. A TCP session must be established, so the IP address used to peer with must be reachable.

In Figure 6-2, Routers A and B are EBGP peers. Routers B, C, and D are IBGP peers.

BGP Next-Hop Selection

The next hop for a route received from an EBGP neighbor is the IP address of the neighbor that sent the update.

When a BGP router receives an update from an EBGP neighbor, it must pass that update to its IBGP neighbors without changing the next-hop attribute. The next-hop IP address is the IP address of an edge router belonging to the next-hop autonomous system. Therefore, IBGP routers must have a route to the network connecting their autonomous system to that edge router. For example, in Figure 6-3, RtrA sends an update to RtrB, listing a next hop of 10.2.2.1, its serial interface. When RtrB forwards that update to RtrC, the next-hop IP address will still be 10.2.2.1. RtrC needs to have a route to the 10.2.2.0 network to have a valid next hop.

To change this behavior, use the neighbor [ip address] next-hop-self command in BGP configuration mode. In Figure 6-3, this configuration goes on RtrB. After you give this command, RtrB advertises its IP address to RtrC as the next hop for networks from AS 65100, rather than the address of RtrA. Thus, RtrC does not have to know about the external network between RtrA and RtrB (network 10.2.2.0).

BGP Next Hop on a Multiaccess Network

On a multiaccess network, BGP can adjust the next-hop attribute to avoid an extra hop. In Figure 6-3, RtrC and RtrD are EBGP peers, and RtrC is an IBGP peer with RtrB. When C sends an update to D about network 10.2.2.0, it normally gives its interface IP address as the next hop for D to use. But because B, C, and D are all on the same multiaccess network, it is inefficient for D to send traffic to C, and C to then send it on to B. This process unnecessarily adds an extra hop to the path. So, by default, RtrC advertises a next hop of 10.3.3.3 (RtrB's interface) for the 10.2.2.0 network. This behavior can also be adjusted with the neighbor [ip address] next-hop-self command.

BGP Synchronization Rule

The BGP synchronization rule requires that when a BGP router receives information about a network from an IBGP neighbor, it does not use that information until a matching route is learned via an IGP or static route. It also does not advertise that route to an EBGP neighbor unless a matching route is in the routing table. In Figure 6-3, if RtrB advertises a route to RtrC, then RtrC does not submit it to the routing table or advertise it to RtrD unless it also learns the route from some other IGP source.

Recent IOS versions have synchronization disabled by default. It is usually safe to turn off synchronization when all routers in the autonomous system run BGP. To turn it off in earlier IOS versions, use the command no synchronization under BGP router configuration mode.